This invention relates to method and apparatus for detecting differences or non-uniformity in wall thickness around the perimeter of hollow translucent objects such as glass or plastic bottles or plastic tubing or the like.
In the past, apparatus has been proposed for measuring the wall thickness of glass or plastic bottles by shining one or more light beams through the bottle, sensing the intensity of the light beam with a photocell as it emerges from the bottle, and comparing the output of the photocell to a predetermined reference level which signifies a desired wall thickness for the bottle. Such apparatus is disclosed in U.S. Pat. No. 3,439,178 to H. R. Rottmann. However, such prior art apparatus has several drawbacks. In the first place, to measure the entire periphery of the bottle it is necessary to rotate the bottle through 360.degree.while the light beam is shining through the bottle. This requires that the bottle be held in one axial position opposite the light beam while the bottle is rotated. This substantially slows down the rate at which the bottle can be processed. Also, the requirement that the bottles be rotated makes such prior art apparatus inapplicable to hollow objects which cannot be rotated, for example, plastic tubing which is being continuously extruded from a ring-shaped extrusion die. Moreover, such prior art apparatus is based on the theory that the opacity of the hollow object under measurement is directly proportional to its wall thickness. This may be true in some applications, but in the case of plastic tubing which is continuously extruded from a die, it has been found that the opacity of the tubing varies from batch to batch of plastic without regard to the wall thickness of the tube. The assumption that opacity is proportional to wall thickness only holds for adjacent regions of tubing which are close enough together to have been formed from substantially similar raw materials under substantially similar conditions.
Because of the aforementioned problems, most commercial plastic tube forming operations make no attempt to scan the tubing as it is formed, but simply measure the wall thickness of completed portions of the tube with a caliper or the like and make adjustments in the extruding die in accordance with such measurements. This is a time-consuming process and large quantities of tubing are formed before the measurement is taken. Accordingly, where unacceptable variations in wall thickness are discovered, this technique results in considerable waste.